A computer network is a collection of interconnected computing devices that can exchange data and share resources. In a packet-based network, such as the Internet, the computing devices communicate data by dividing the data into small blocks called packets, which are individually routed across the network from a source device to a destination device. The destination device extracts the data from the packets and assembles the data into its original form. Dividing the data into packets enables the source device to resend only those individual packets that may be lost during transmission.
Certain devices, referred to as routers, maintain routing information that describes available routes through the network. Each route defines a path between two locations on the network. Upon receiving an incoming packet, the router examines information within the packet and forwards the packet in accordance with the routing information. In order to maintain an accurate representation of a network, routers maintain control-plane peering sessions through which they exchange routing or link state information that reflects the current topology of the network.
Routers typically send periodic packets to each other via the peering sessions to confirm connectivity and to indicate operational status of each device. These periodic packets are sometimes referred to as “keepalives” or “hellos.” For example, a router may send periodic packets to a peer router every 50 milliseconds (ms) to indicate that the router is still operational. Likewise, the router may detect reception of corresponding periodic packets from the peer router within the same period of time (e.g., 50 ms). When a packet is not received in the allotted time frame, the router determines that a network event has occurred, such as failure of the peer router or failure of a link or node connecting the two routers. Consequently, the router may update various routing information to redirect network traffic and may issue a number of routing protocol update messages to neighboring routers indicating a topology change.
As one example, routers may exchange periodic packets by establishing a session provided by the bidirectional forwarding detection (BFD) protocol. In accordance with BFD, a first router periodically sends BFD packets at a negotiated transmission time interval and detects a connectivity failure when the router does not receive a BFD packet from a second router within session detection time interval. The transmission time interval for BFD may be negotiated by the routers. The BFD session detection time interval is not negotiated. Instead, each receiving router independently calculates BFD session detection interval based on the negotiated transmit interval and a detection multiplier statically configured for that router. For instance, a router may negotiate to receive BFD packets every 50 ms from a peer router and may independently utilize a detection multiplier of three (3) times that interval, i.e., 150 ms in this example, for detecting failure. If the receiving router does not receive a BFD packet from the peer router within the 150 ms session detection time interval, the receiving router detects a connectivity failure with respect to the second router. Consequently, the receiving router may update its routing information to route traffic around the second router. Further details of the BFD protocol may be found in the proposed standard for BFD, by D. Katz and D. Ward (Juniper Networks, June 2010, ISSN: 2070-1721), the entire content of which is incorporated herein by reference.